The present invention relates to methods and apparatus for purifying inert gas streams and more particularly for removing parts per million levels of impurities from nitrogen-based streams.
As the semiconductor industry is developing integrated circuits with ever increasing line densities, the manufacturing processes employed require that materials utilized are as free of impurities as is possible. Inert gas such as nitrogen or argon, etc., is frequently utilized in semiconductor manufacturing processes and while commercially available nitrogen and argon are relatively pure, it is necessary to assure that even greater purities are maintained so as to avoid contamination of semicondiuctor materials by impurities such as H.sub.2, H.sub.2 O, CO, CO.sub.2, O.sub.2, etc.
Although nitrogen will react with certain elements under particular conditions, it will be understood that the term "inert gas" as used herein includes nitrogen. It has previously been proposed to remove certain of the above-mentioned impurities, e.g., O.sub.2 by catalytically combining oxygen with hydrogen over a catalyst such as DeOxo D, etc. However, this process requires that relatively high temperatures such as 450.degree. C. be employed to assure that such catalytic combustion occurs to the necessary extent. It is then necessary to cool the hot "purified" inert gas in heat exchangers or other suitable equipment. The latter is relatively expensive and adds a significant step to the overall purification process. A typical catalytic process for reacting hydrogen with oxygen is disclosed in Japanese published patent application No. 59-54608.
In addition, it is known to use zeolites to adsorb oxygen in inert gas streams to thereby purify the latter. This typically involves cooling a zeolite bed to a very low temperature, i.e. below about -220.degree. F. or so and essentially requires that cryogenic conditions be established. This in turn will require that special materials and insulation, etc., be provided. A typical adsorption process of this type is disclosed in U.S. Pat. No. 3,928,004. It is also known to utilize zeolites to remove CO.sub.2 from air or inert gas streams at ambient temperatures and such a process is illustrated in U.S. Pat. No. 3,885,927. Although it would appear from this reference that carbon dioxide can be removed from air or inert gas streams, it does not appear that such adsorbents as disclosed therein are effective to remove other impurities such as O.sub.2, H.sub.2, CO, etc., and no such capability is suggested by this patent.
Other techniques for removing oxygen from gas streams include the use of copper-based getter materials as is described in Japanese published patent application No. 53-33312. In this process, the getter material is heated to a temperature of at least 150.degree. C. Subsequently, the gas stream is cooled and this process only effective to remove oxygen and not other impurities such as H.sub.2 O and CO.sub.2, etc.
Consequently, none of the processes developed to date appear to be capable of removing a wide range of impurities from inert gas streams in a relatively simple and inexpensive manner at approximately ambient temperature to which end the present invention is directed. Furthermore, there are no known prior art processes which are effective to remove parts-per-million levels of CO from inert gas streams under ambient temperature conditions.